The present invention relates to protection against leakage currents to ground, particularly in an electric range employing sheathed electrical resistance heating units.
In electrical load devices supplied through conductors from a power source there is a particular failure mode known as a ground fault where current flows between one of the "hot" conductors and ground. To alleviate the problems posed by such ground faults, various devices known as ground fault interrupters (GFI's) have been developed and are commercially available. A GFI senses any minute leakage current flowing within a load device from a line "hot" conductor to ground. When such leakage current is sensed, current to the load device is immediately interrupted, thereby avoiding a shock hazard. A power relay typically does the actual interrupting. The specific way in which a ground fault condition is usually sensed is by employing a differential current transformer to detect a current imbalance in the power input lines. If the current flowing into the load does not exactly equal the current flowing out of the load, then it is presumed that some of the current is diverting to ground. Examples of such ground fault interrupters are disclosed in Vassos et al U.S. Pat. No. 3,633,070 and Legatti et al U.S. Pat. No. 3,899,717. The ground fault interrupters disclosed in both of these patents also include time delay means.
It will be appreciated that most power sources have some form of overcurrent protection. This overcurrent protection may be provided by a fuse or circuit breaker which serves to interrupt the power source when a predetermined current threshold is exceeded. In order to prevent false or nuisance tripping as a result of momentary overloads, many fuses and circuit breakers have a time delay, with the amount of time delay before actual circuit interruption usually inversely related to the amount of overcurrent. Accordingly, an overcurrent protection means associated with a power source must be capable of reliably interrupting relatively large fault currents, substantially in excess of the predetermined current threshold. It is with such an overcurrent-protected power source that the present invention is intended to operate.
More particularly, the present invention is for use with load devices which are supplied from such an overcurrent-protected power source and which are subject to two general classes of failure. The first class of failure may conveniently be generally described as encompassing relatively low current ground fault failures more particularly characterized by excessive current flow between at least one of the conductors and a ground reference point and ground, with circuit remaining at or below the predetermined current threshold of the power source overcurrent protection means. The second class of failure may conveniently be generally described as encompassing relatively high current failures, which includes high-current ground fault failures, more particularly characterized by current through at least one of the conductors being above the predetermined current threshold of the power source overcurrent protection means. Each of these classes of failure is described in greater detail below in the particular context of an electric range.
Specifically, a typical heating unit in an electric range is a sheathed electrical resistance heating unit comprising a heating element in the form of a spiralled electrical resistance wire encased in an elongated ceramic-filled metal outer sheath which is electrically conductive and connected to the frame of the range. The ceramic material, typically magnesium oxide, transmits heat, but is an electrical insulator. Thus, the outer sheath becomes hot, but normally remains electrically insulated from the heating element.
One particular failure mode possible in such a sheathed electrical resistance heating unit is associated with a breakdown in the insulation qualities of the magnesium oxide, permitting current to flow between the heating element and the outer sheath. Typically, such a failure begins gradually, drawing relatively little current initially. This initial failure stage may be termed an incipient ground fault. However, if not immediately interrupted, a destructive high current arc may ensue.
It should be mentioned that, particularly in the context of a sheathed electrical resistance heating unit, it is possible to in effect directly sense leakage current to ground and interrupt the same without employing a differential current transformer. Such an approach is implemented in the arrangement disclosed in commonly-assigned U.S. Pat. No. 4,044,224, issued to Jenkins and Herbst, wherein the direct electrical connection between the outer conductive sheath and ground of the dishwasher heating unit is interrupted by a fusible link. During normal operation of the heating unit when the ceramic insulation material is intact, substantially no current flows through the outer sheath ground connection (with the exception of a small amount of AC leakage current largely as a result of capacitive and moisture effects). However, when a fault occurs, significant current flows through the outer sheath ground connection, causing the fusible link to open. Additionally, in the Jenkins and Werbst arrangement, a switch is mechanically arranged to open when the fusible link is broken, and this switch cuts off power to the heating element.
Accordingly, it would appear desirable to employ a ground fault interrupter type device in combination with a range to sense a ground fault type failure or an incipient ground fault type failure of the sheathed electrical resistance heating unit. In many cases such a failure, and particularly an incipient failure, is accompanied by only a slight increase in the current drawn from the supply, and hence is a failure of the first class as described above. In many instances, by disconnecting the power, the range is safe to operate again because the failed element remains inoperative but presents no safety hazard. Should a ground fault persist in the failed element, the circuit opens again.
A device such as a range is also subject to the second class of failure mentioned above characterized by a much higher current than a typical incipient ground fault. Such a failure of the second class may be a major arcing ground fault, a short circuit between a conductor anywhere in the range and ground where insulation may have deteriorated, or other type of short circuit. Since a household electric range is a relatively high current device, typically operating from a 40 or 50 ampere circuit, at 240 volts, substantial fault current may occur. In typical household range circuit protected by a 50 ampere circuit breaker, a momentary fault current of 3,000 amperes may sometimes occur. As a result, in the absence of the present invention, the circuit interrupting portion of any ground fault protection device for use in combination with an electric range would require circuit interrupting contacts sufficient to interrupt the maximum fault current which might conceivably occur. This is because any major high-current fault may in fact be, or be accompanied by, a ground fault, which would trigger the ground fault interrupter device. Therefore, to merely include a ground fault interrupter in a range, without the present invention, would lead to a requirement to provide relatively heavy contacts on any device employed as the current-interrupting element of the ground fault interrupter, with an attendant relatively high cost.